How can I add a single curly brace to a macro?

Question

I am trying to compose \textit{abc} like this:

\documentclass{article}
\makeatletter

\def\test{}

\catcode123=12 % change cat code of "{" to "other"
\g@addto@macro\test{
\catcode123=1  % restore the cat code

\g@addto@macro\test{abc}

\catcode125=12 % change cat code of "}" to "other"
\g@addto@macro\test}
\catcode125=2  % restore the cat code

\begin{document}
    \test
\end{document}

However, issuing \test does not print an italic abc. How can I achieve what I am trying to do?

Answer 1

Although expl3's regex engine (which is loaded by regexpatch) is an excellent tool (and my job description is to merchandise expl3), in this case it's like using a shotgun to kill a fly :-)

You can achieve the same effect using a macro to define another. A simplistic definition of a \MakeItalic macro would be \def\MakeItalic#1{\edef#1{\noexpand\textit{#1}}}. The \noexpand would make sure that \textit doesn't blow up inside the \edef (you could omit the \noexpand if you used \protected@edef instead). The #1 would be your macro that contains abc, which would expand into abc.

A more robust definition of \MakeItalic would require e-TeX (you're using LaTeX, so that is the default anyway):

\newcommand{\MakeItalic}[1]{%
  \edef#1{\noexpand\textit{\unexpanded\expandafter{#1}}}}

The difference from the previous version is that instead of completely expanding #1, we expand it only once with \unexpanded\expandafter{#1}. This makes sure that if your \test macro contains other macros or active characters it won't explode.

A test document:

\documentclass{article}
\newcommand{\MakeItalic}[1]{%
  \edef#1{\noexpand\textit{\unexpanded\expandafter{#1}}}}
% Complicated definition of \test
\def\test{abc}
% Add \textit{...}
\MakeItalic\test
\begin{document}
\texttt{\meaning\test} = \test
\end{document}

produces:

Answer 2

You could use the following patterns to build your macro's replacement text. The basic idea is to not build the final result directly but to first create a sequence of commands that yields the final result when it is fully expanded in an \edef. For building the sequence you can use the \g@addto@macro macro, adding material as your algorithm goes.

Here are the sequence patterns to be added for various aspects (see this answer for an explanation of the brace patterns):

• Add a macro name: \noexpand\macroname
• Add general material that should occur unexpanded: \unexpanded{...}
• Add a left brace: {\iffalse}\fi
• Add a right brace: \iffalse{\fi}
• All unexpandable material or \protect'ed macros can be used directly

From those pattern we can build the macro mentioned in your question:

\documentclass{article}
\makeatletter

\def\test{}

\g@addto@macro\test{\noexpand\textit}
\g@addto@macro\test{{\iffalse}\fi}
\g@addto@macro\test{abc}
\g@addto@macro\test{\iffalse{\fi}}

%\show\test
\edef\test{\test}
%\show\test

\begin{document}
    \test
\end{document}

If you uncomment the \show calls in this example, you can see the macro definition before

> \test=macro:
->\noexpand \textit {\iffalse }\fi abc\iffalse {\fi }.

and after the expansion

> \test=macro:
->\textit {abc}.

Answer 3

One answer could look like this:

\documentclass{article}
\usepackage{regexpatch}

\def\test{abc}
\regexpatchcmd{\test}{.*}{\c{textit}\cB\{\0\cE\}}{}{}

\begin{document}
    \test
\end{document}

A variant:

\documentclass{article}
\usepackage{regexpatch}
\makeatletter

\def\test{\textit XXX}
\g@addto@macro\test{abc}
\g@addto@macro\test{XXX}

\regexpatchcmd{\test}{XXX(.*)XXX}{\cB\{\1\cE\}}{}{}

\begin{document}
    \test
\end{document}